1. Technical Field
The present disclosure relates to a technique of determining whether a pure tone has been heard in comfort. More specifically, the present disclosure relates to an uncomfortable sound pressure determination system for determining, when listening to a pure tone, a sound pressure that is felt so loud as to be uncomfortable; and the like.
2. Description of the Related Art
In recent years, people suffering from presbycusis are increasing in number due to the aging society. Due to the increased opportunities for listening to loud music for long hours as well as other influences, there is an increasing number of young people suffering from hypacusia associated with acoustic traumas. Moreover, due to the downsizing and improved performance of hearing aids, users have come to wear hearing aids with less of a psychological barrier. This has led to an increasing number of users wearing hearing aids.
A user suffering from hypacusia has difficulty in hearing sounds of a specific frequency(s). This specific frequency varies from user to user. A hearing aid amplifies the amplitude of a sound signal at this specific frequency, thus making it easier for the user to hear sounds.
A hearing aid is required to change the amount by which it amplifies sounds, in accordance with the level of deterioration in the hearing of the user. Therefore, before beginning use of a hearing aid, “fitting” is required for adjusting the amount of sound amplification in accordance with the hearing of each user.
The purpose of fitting is to keep the output sound pressure of a hearing aid at an MCL (most comfortable level). As used herein, the “output sound pressure” of a hearing aid refers to the fluctuations in air pressure that are perceivable to humans as a sound. The MCL defines a sound pressure which guarantees comfortable hearing by the user. The hearing aid needs to ensure that the output sound pressure satisfies MCL for each sound frequency.
Examples of inappropriate fitting may be: (1) an insufficient amount of amplification for sound pressure; or (2) an excessive amount of amplification for sound pressure. For example, if the amount of amplification for sound pressure is insufficient, the user cannot aurally distinguish audios. In this case, the aforementioned purpose of using a hearing aid is not met. If the amount of amplification for sound pressure is excessive, the user is capable of aural distinction of audios, but may find the audio to be loud, which prevents the user from using the hearing aid over a long time. Therefore, a fitting of a hearing aid needs to be done in such a manner that neither (1) nor (2) occurs. Especially (2) possesses a possibility that the hearing aid may present an audio with an unduly high sound pressure to the user. This has created danger of hurting the user's ear with audios having high sound pressure.
Fitting generally comes into two steps. A first step of fitting is measuring an audiogram. An “audiogram” refers to a measurement of a threshold value (hearing threshold level: HTL) defining the smallest sound pressure of a pure tone that allows it to be heard by a user. An audiogram may be, for example, a diagram in which such a threshold value (decibel value) is plotted for different frequencies (e.g., 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz).
A second step of fitting is determining an amount of amplification for sound pressure. For example, by using a mathematical function (called a fitting theory) for estimating an amount of sound amplification, an amount of amplification is determined for each frequency and for each sound pressure of an input sound. There are a number of types of fitting theories, for example: the half-gain method, in which an insertion gain of each frequency is made half of the threshold value of that frequency; Berger's method, which slightly augments the amplify from 1000 Hz to 4000 Hz by taking into consideration the frequency band and level of conversational voices; the POGO method which, based on the half-gain method, reduces the gains at 250 Hz and 500 Hz (where there is not so much speech sound information but a lot of noise component is included) by 10 dB and 5 dB, respectively; and the NAL-R method, which performs amplification so that a frequency of long-term sound analysis of words will fall around a comfortable level.
Moreover, the “fitting theory” is also inclusive of a method of determining an amount of amplification for sound pressure by utilizing the information of a threshold value, a UCL (uncomfortable level) which is a high sound pressure level that is felt uncomfortable to the user, and the MCL. In that case, before determining an amount of amplification for sound pressure, the UCL and MCL are either measured or estimated. In order to avoid problem (2) above, it is necessary to measure the UCL, and set an amount of amplification in a range such that the UCL is not exceeded.
Similarly to audiogram measurement, a UCL is to be measured for each frequency. Conventionally, the UCL is measured based on subjective reporting. “Subjective reporting” involves, after a user hears a sound, the user making a subjective account as to how the sound was felt to him or her. For example, while using an audiometer, continuous sounds or discontinuous sounds are presented to the user by using an ascending method (i.e., the sound pressure level is gradually increased), and the user is asked to report whether the sound pressure is so loud that he or she cannot tolerate hearing it for a long time. Then, a sound pressure beyond which the user cannot retain tolerance over a long time, according to their own reporting, is defined as a UCL (Takashi KIMITSUKI et al., “Inner ear auditory testing in patients with normal hearing showing hyperacusis”, 2009; hereinafter “Non-Patent Document 1”).
An UCL measurement through subjective reporting is difficult because the UCL criterion will fluctuate under individual influences or the influences of linguistic expressions, and thus there is no established technique. Therefore, methods of taking an objectively measurement of UCL by using electroencephalogram are under development. For example, in a technique disclosed in Thornton, A. R. et al., “The objective estimation of loudness discomfort level using auditory brainstem evoked responses”, 1987 (hereinafter “Non-Patent Document 2”), a UCL is estimated based on a relationship between the stimulation intensity and the latency of a V wave that is contained in a brainstem response called ABR (auditory brainstem response). As the sound pressure increases, the V wave latency decreases. The sound pressure of the sound which the user was hearing when the decrease in V wave latency became saturated is identified. A sound pressure which is obtained by adding a constant (e.g., 15 or 10) to this identified sound pressure is defined as the UCL.